1. Field of the Invention
The present invention relates to a high voltage, heat resisting electrically insulated coil which has insulating layers prepared by a poly-functional epoxy resin having at least three of p-(2,3-epoxy propoxy) phenyl groups in the molecule, and an electric rotating machine using the coil, and also relates to a method of manufacturing same.
2. Description of the Prior Art
Currently, the improvement in technology of insulation treatment has been remarkable, especially, the impregnation method of thermosetting resin of the solventless type has been rapidly developed in the field of manufacturing of high voltage electric rotating machines used under severe conditions such as a direct current motor for an electric car and for general industries in view of (1) high electrical reliability (2) good heat dissipation (3) high moisture resistance. Recently, as the increasing in capacity and the decreasing in weight and in size of electric machinery such as an electric rotating machine, the development of an insulating system which enables a rotating machine be operate for a long period continuously under the H-class condition (180.degree. C.) or higher class. The properties of thermosetting resin composition of the solventless type which are necessary for impregnation of rotating machine coils are (1) low viscosity (less than 10 poises at impregnating time) to make the impregnation easy, (2) no generation of volatile substances to prevent void formation in a process of heating and hardening of the resin composition, (3) superior electrical and mechanical characteristics, (4) high compatibility with insulating base sheets, and (5) high heat resistance, i.e. the characteristics of thermal stability of the cured resin in short and long period is 180.degree. C. or above.
Generally, there is a dilemma due to the molecular structure of the resin between lowering viscosity and improving heat resistance, so that molecular design of the resin is difficult. As for solventless type thermosetting resin composition for impregnation of coils of an electric rotating machine, epoxy resin compositions comprising bi-functional epoxy resins and acid anhydride are generally quoted as described in Japanese Patent Application Laid-Open No. 60-5210(1985). But, the rotating machine coils manufactured by impregnation of the epoxy resin compositions mentioned above and heat hardening them, can not be used at H-class or above class condition because depletion in mechanical and electric insulating property will begin after long term operation under F-class (temperature at 155.degree. C. or above) condition.
Generally, as for the method to improve heat resistant property of the solventless type thermosetting resin composition, a method to introduce heterorings in the molecular structure such as in maleimide has conventionally been used. But, as the viscosity is increased by introducing the heterorings into the molecular structure of the resin, there is no resin which satisfies the demand for a resin having low viscosity and high heat resistance concurrently. In view of this point, a combination of maleimide and epoxy resin has been attempted. But, there have been problems such as lowering of heat resistance, yielding of precipitates during storage, generation of stimulative odor from the resin composition.
Recently, compositions comprising poly-functional epoxy resin having at least three functions and acid anhydrides as described in Japanese Patent Publication No. 57-13571(1982), No. 57-14763(1982), No. 62-1648(1987), No. 62-44767(1987), and in Japanese Patent Application Laid-Open No. 61-252224(1986), No. 64-4615(1989), No. 1-4615(1989) etc. have been attracting interests because of their relatively low cost, low viscosity before hardening, and high heat resistance after hardening. The Japanese Patent Application Laid-Open No. 59-12061(1984) discloses a pre-preg mica sheet using tetre-functional epoxy resin. The mica sheet is generally used to form insulating layers by winding it around a conductor, followed by hardening therein.
Mica is indispensable in the manufacturing of high voltage coils for high voltage rotating machines which are operated under severe conditions such as a direct current motor for an electric car and for general industry. But, mica itself does not have enough mechanical strength to be wound around a conductor. Therefore, mica has to be reinforced with a backing film such as glass cloth, kapton, etc., by bonding mica with a binding resin. Hitherto, as for the binding resin, bi-functional epoxy resins for general use, phenol novolak type epoxy resins, and silicone resins have been used. But, an electrically insulated coil obtained by winding an insulating base sheet which is reinforced by the binding resin around a conductor, and impregnating the wound sheet with a thermosetting resin composition, especially the poly-functional epoxy resin composition, followed by hardening the impregnated resin, have problems such that decreasing of thermal conductivity of the insulating layer due to peeling off the layer and a rapid increment in temperature of the electrically insulated coil lead to breakdown of the insulating layer.